Orthogonal frequency division multiplexing (OFDM) is a multi-carrier multiplexing scheme that partitions a frequency band (e.g., the system-bandwidth) into multiple (N) orthogonal subbands. These subbands are also called tones, subcarriers, bins, and so on. With OFDM, each subband is associated with a respective subcarrier that may be independently modulated with data.
OFDM has certain desirable characteristics such as high spectral efficiency and robustness against multipath effects. However, a major drawback with OFDM is a high peak-to-average power ratio (PAPR), which means that the ratio of the peak power to the average power of an OFDM waveform can be high. The high PAPR for the OFDM waveform results from possible in-phase addition of all the subcarriers when they are independently modulated with data. In fact, it can be shown that the peak power can be up to N times greater than the average power.
The high PAPR for the OFDM waveform is undesirable and may degrade performance. For example, large peaks in the OFDM waveform may cause a power amplifier at the transmitter to operate in a highly non-linear region or possibly clip, which would then cause intermodulation, distortion and other artifacts that can degrade signal quality. The degraded signal quality may adversely affect performance for channel estimation, data detection, and so on.
There is therefore a need in the art for a transmission scheme that provides good performance and does not have a high PAPR.